aye (interj.)

also *ayu-, Proto-Indo-European root meaning "vital force, life; long life, eternity."

It forms all or part of: age; aught (n.1) "something; anything;" aye (adv.) "always, ever;" Ayurvedic; coetaneous; coeval; each; eon; eternal; eternity; ever; every; ewigkeit; hygiene; longevity; medieval; nay; never; no; primeval; sempiternal; tarnation; utopia. 

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit ayu- "life;" Avestan aiiu "age, life(time);" Greek aiōn "age, vital force; a period of existence, a lifetime, a generation; a long space of time," in plural, "eternity;" Latin aevum "space of time, eternity;" Gothic aiws "age, eternity," Old Norse ævi "lifetime," German ewig "everlasting," Old English a "ever, always."  

12c., a shortening of Old English ic, the first person singular nominative pronoun, from Proto-Germanic *ek (source also of Old Frisian ik, Old Norse ek, Norwegian eg, Danish jeg, Old High German ih, German ich, Gothic ik), from PIE *eg- "I," nominative form of the first person singular pronoun (source also of Sanskrit aham, Hittite uk, Latin ego (source of French Je), Greek ego, Russian ja, Lithuanian aš).

Reduced to i by mid-12c. in northern England, later everywhere; the form ich or ik, especially before vowels, lingered in northern England until c. 1400 and survived in southern dialects until 18c. It began to be capitalized mid-13c. to mark it as a distinct word and avoid misreading in handwritten manuscripts.

The reason for writing I is ... the orthographic habit in the middle ages of using a 'long i' (that is, j or I) whenever the letter was isolated or formed the last letter of a group; the numeral 'one' was written j or I (and three iij, etc.), just as much as the pronoun. [Otto Jespersen, "Growth and Structure of the English Language," p.233]

The dot on the "small" letter -i- began to appear in 11c. Latin manuscripts to distinguish the letter from the stroke of another letter (such as -m- or -n-). Originally a diacritic, it was reduced to a dot with the introduction of Roman type fonts. The letter -y- also was written with a top dot in Old English and early Middle English, during the centuries when -i- tended to be written with a closed loop at the top and thus was almost indistinguishable from the lower-case thorn (þ). In names of U.S. highways (by 1966) it is short for Interstate (adj.).