英语词源字典 - 在线词源网站

diacritic (adj.)[diacritic 词源字典]: 1690s, of sounds, from Greek diakritikos "that separates or distinguishes," from diakrinein "to separate one from another," from dia- (see dia-) + krinein "to separate, decide, judge" (see crisis). As a noun, from 1866. Related: Diacritical.[diacritic etymology, diacritic origin, 英语词源]
tittle (n.): late 14c., "small stroke or point in writing" (Wyclif, in Matt. v:18), translating Latin apex in Late Latin sense of "accent mark over a vowel," which itself translates Greek keraia (literally "a little horn"), used by the Greek grammarians of the accents and diacritical points, in this case a Biblical translation of Hebrew qots, literally "thorn, prick," used of the little lines and projections by which the Hebrew letters of similar form differ from one another.

Wyclif's word is borrowed from a specialized sense of Latin titulus (see title (n.)), which was used in Medieval Latin (and in Middle English and Old French) to indicate "a stroke over an abridged word to indicate letters missing" (and compare Provençal titule "the dot over -i-").
"As apex was used by the Latin grammarians for the accent or mark over a long vowel, titulus and apex became to some extent synonymous; hence Wyclif's use of titil, titel to render L. apex" [OED]
Compare tilde, which is the Spanish form of the same word.