12c. shortening of Old English ic, first person singular nominative pronoun,
from Proto-Germanic *ekan
(cognates: Old Frisian ik, Old Norse ek, Norwegian eg, Danish jeg, Old High German ih, German ich, Gothic ik),
from PIE *eg-, nominative form of the first person singular pronoun
(cognates: 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, it began to be capitalized mid-13c. to mark it as a distinct word and avoid misreading in handwritten manuscripts.