expression, godan verb (-ku)
write down (in advance); jot down (for later)
Colloquial contraction of 書いておく, used in casual speech and writing. Implies writing something down now for future convenience or as a preparatory action.
忘れないうちに、メモに書いとくね。
I'll jot it down in a memo before I forget.
明日
I wrote down the shopping list for tomorrow.