Originally from 尊い (toutoi), meaning
precious, valuable or priceless, it became Japanese internet slang "ていてい (or てぇてぇ)" roughly meaning someone else'
s actions are wholesome, or
precious.
Popularised in the west by subtitled V-Tuber clips, where it was used typically during moments where the V-Tubers were acting affectionate to one another; this hard to translate word was often simply romanised. This resulted in several spellings, the most popular of which seems to be "TeeTee", whith other variant such as: tee-tee, tee tee, tei-tei,
TT and te-te.