(not) at all; completely; utterly — used with negative sentences to emphasize lack or absence
彼の冗談はてんで面白くない。
His jokes aren't funny at all.
テストはてんでダメだった。
I did terribly on the test (completely hopeless).
その話はてんで信じられない。
That story is utterly unbelievable.
very; extremely — used colloquially to emphasize degree (in affirmative contexts)
彼女はてんでかわいいね。
She's really cute, isn't she?
今年の夏はてんで暑かった。
This summer was extremely hot.