英语词源字典 - 在线词源网站

underpin (v.)[underpin 词源字典]: "support or prop," 1520s (figurative); 1530s (literal), from under + pin (v.). Related: Underpinned; underpinning.[underpin etymology, underpin origin, 英语词源]
underpinning (n.): late 15c., "action of supporting or strengthening from beneath," from under + present participle of pin (v.). Figurative sense of "prop, support" is recorded from 1580s.
underprivileged (adj.): 1896, from under + past participle of privilege (v.). Noun use (short for underprivileged persons) is attested from 1935.
underrate (v.): also under-rate, 1640s, "to esteem at too little worth," from under + rate (v.). Related: Underrated; underrating.
underscore (v.): 1771, "to draw a line under," from under + score (v.). The figurative sense of "to emphasize" is attested from 1891. Noun meaning "a line drawn below (something)" is recorded from 1901.
undersea (adj.): 1610s, from under + sea.
underserve (v.): "to serve insufficiently," 1710, from under + serve (v.). Related: Underserved; underserving.
undershirt (n.): 1640s, from under (adj.) + shirt (n.). Similar formation in North Frisian onnersjürt, Danish underskjorte. Old English had undersyrc (see sark (n.)).
undershoot (v.): 1660s, "to shoot too low," from under + shoot (v.). In reference to aircraft or pilots, recorded from 1918. Undershot as a type of water wheel is recorded from c. 1600.
underside (n.): 1680s, from under (adj.) + side (n.). Similar formation in Dutch onderzijde, Danish underside, German unterseite.
undersign (v.): 1570s, from under + sign (v.). Related: Undersigned; undersigning.
understand (v.): Old English understandan "comprehend, grasp the idea of," probably literally "stand in the midst of," from under + standan "to stand" (see stand (v.)). If this is the meaning, the under is not the usual word meaning "beneath," but from Old English under, from PIE *nter- "between, among" (cognates: Sanskrit antar "among, between," Latin inter "between, among," Greek entera "intestines;" see inter-). Related: Understood; understanding.

That is the suggestion in Barnhart, but other sources regard the "among, between, before, in the presence of" sense of Old English prefix and preposition under as other meanings of the same word. "Among" seems to be the sense in many Old English compounds that resemble understand, such as underniman "to receive," undersecan "examine, investigate, scrutinize" (literally "underseek"), underðencan "consider, change one's mind," underginnan "to begin." It also seems to be the sense still in expressions such as under such circumstances.

Perhaps the ultimate sense is "be close to;" compare Greek epistamai "I know how, I know," literally "I stand upon." Similar formations are found in Old Frisian (understonda), Middle Danish (understande), while other Germanic languages use compounds meaning "stand before" (German verstehen, represented in Old English by forstanden "understand," also "oppose, withstand"). For this concept, most Indo-European languages use figurative extensions of compounds that literally mean "put together," or "separate," or "take, grasp" (see comprehend). Old English oferstandan, Middle English overstonden, literally "over-stand" seem to have been used only in literal senses. For "to stand under" in a physical sense, Old English had undergestandan.
understandable (adj.): late 14c., "able to understand;" late 15c., "able to be understood," from understand + -able. Related: Understandably.
understanding (n.): Old English understanding "comprehension," verbal noun from understand (v.). Meaning "mutual agreement" is attested from 1803.
understate (v.): 1781, from under + state (v.). Related: Understated; understating.
understated (adj.): 1939, of clothing, fashions, writing, etc., figurative use of the past participle of understate (v.).
understatement (n.): 1799, from under + statement.
understory (n.): in reference to forest vegetation, also under-story, 1902, from under + story (n.).
understudy (v.): also under-study, 1852, in the theatrical sense "memorize a part so as to be capable of performing on short notice it in the absence of the one to which it is assigned," from under + study (v.). The noun is attested from 1848, translating Italian supplimento.
undertake (v.): c. 1200, "to entrap;" c. 1300, "to set about (to do)," from under + take (v.). Similar formation in French entreprendre "to undertake," from entre "between, among" + prendre "to take." The under in this word may be the same one that also may form the first element of understand. Old English had underniman "to trap, accept" (cognate with Dutch ondernemen, German unternehmen).