Entirely, Truly, Really, Wholly
This is the adverb version of the kanji all. So, it's like you add an 'ly' to it. But, that doesn't work too well (allly? c'mon). So, instead think of this word as completely, entirely or truly.
Since this word consists of a kanji with hiragana attached, you can bet that it will use the kun'yomi reading. You didn't learn that reading with this kanji, so here's a mnemonic to help you:
The person who completely doesn't understand this vocab word is Matt (まった). Be sure to take note of the small っ, too.
cannot do at all
to have no idea, to not understand at all
to not know at all
I have the exact idea as you.
There's no weekdays off at all.
I pulled the lever with all my strength, but there was no water coming out.