Hand-in-hand attested from c. 1500 as "with hands clasped;" figurative sense of "concurrently" recorded from 1570s. underhanded (adj.) in reference to a throw, etc., "performed or done with the knuckles turned under," 1807, from under + hand (n.).

Where does the phrase underhanded come from?

Underhanded itself comes is the adjective form of underhand, which was first used in Old English to mean 'in subjugation, under the hand of an authority or ruler' before 900 CE.

What does underhanded mean?

: marked by secrecy, chicanery, and deception : not honest and aboveboard : sly an underhanded attempt to gain power.