How did the two-part compound 打坐 come to mean seated meditation? What is the function of the 打 in that compound?
According to the 《佛光大辭典》,
指跏趺而坐 (to sit cross-legged)，使心入定 (to settle the mind)。
即指坐禪 (za-zen, to meditate)。
「打」，動作行為之意 (means to do or to act)。
According to the 《汉语大词典》,
谓从事某种工作 (to do some kind of work) 或做出某种行为 (to make some kind of behavior)。
Therefore, the 打 in 打坐 means to do, and 打坐 is to do the 靜坐 (to sit still with a peaceful mind).
Since the purpose of 打坐 is to settle the mind, it refers to meditation.
(dan chose 打(38) in the 《汉语大词典》.)
One of sense of 打 is:
It denotes the continuity of an action. So, 打坐 means sit and continue it on and on, that is, meditation.
打坐 is a whole word meaning meditation. If you separate the character, it doesn't mean anything related to meditation. 打 as a verb means hit.