grinyoudaoicibaDictYouDict[grin 词源字典]
grin: [OE] Modern English grin and groan are scarcely semantic neighbours, but a possible common ancestor may provide the link: prehistoric Indo-European *ghrei-, which seems to have meant something like ‘be open’. It has been suggested as the source of a range of verbs which started off denoting simply ‘open the mouth’, but have since differentiated along the lines ‘make noise’ and ‘grimace’. Grin has taken the latter course, but close relatives, such as Old High German grennan ‘mutter’ and Old Norse grenja ‘howl’, show that the parting of the semantic ways was not so distant in time.

Old English grennian actually meant ‘draw back the lips and bare the teeth in pain or anger’. Traces of this survive in such distinctly unfunny expressions as ‘grinning skull’, but the modern sense ‘draw back the lips in amusement’ did not begin to emerge until the 15th century. Groan [OE], on the other hand, is firmly in the ‘make noise’ camp.
=> groan[grin etymology, grin origin, 英语词源]
grin (n.)youdaoicibaDictYouDict
1630s, from grin (v.).
grin (v.)youdaoicibaDictYouDict
Old English grennian "show the teeth" (in pain or anger), common Germanic (cognates: Old Norse grenja "to howl," grina "to grin;" Dutch grienen "to whine;" German greinen "to cry"), from PIE root *ghrei- "be open." Sense of "bare the teeth in a broad smile" is late 15c., perhaps via the notion of "forced or unnatural smile." Related: Grinned; grinning.