adverb
(not) at all; completely; utterly
Used with a negative predicate to mean 'not at all' or 'completely not'. Emphatic negative.
彼の言っていることはてんでわからない。
I don't understand what he's saying at all.
あの映画はてんで面白くなかった。
That movie wasn't interesting at all.
adverb
Rare affirmative use meaning 'very' or 'extremely'. Mostly found in older or dialectal speech; modern standard Japanese prefers とても or すごく.
この辺りは昔、てんでにぎやかだったそうだ。
I hear this area used to be very lively.
The origin is uncertain. It may be a sound-symbolic (mimetic) formation, but no clear derivation is known.