Where does “ツイ” come from?

ツイ (Japanese) comes from Japanese ツイッター, from English Twitter, from Middle English twitren, from Old English twiterian, from Proto-Germanic twitwizōną — To twitter; chirp.

Ancestry of “ツイ”, step by step

StepLanguageWordMeaning
1JapaneseツイッターTwitter a social-networking website
2EnglishTwitterA user account belonging to the above service; To...
3Middle Englishtwitren
4Old Englishtwiterian
5Proto-GermanictwitwizōnąTo twitter; chirp
Every word from Proto-Germanic twitwizōnąEvery word from Old English twiterianEvery word from Middle English twitren