Longman Dictionary of Contemporary English
The Collaborative International Dictionary
Hankey-pankey \Han"key-pan"key\, Hankey-pank \Han"key-pank`\(h[a^][ng]"k[y^]*p[a^][ng]"k[y^]), n. [Cf. Hocus-pocus.] [Also spelled hanky-panky.]
Professional cant; the chatter of conjurers to divert attention from their tricks; hence, jugglery. [Colloq.]
Illegal or unethical behavior, usually surreptitious; as, the boss got suspicious when profits seemed lower than expected, and hired an investigator to see if any hankey-pankey was going on. [Informal]
Extramarital sexual relations, especially adultery.
Douglas Harper's Etymology Dictionary
also hanky panky, 1841, "trickery," British slang, possibly a variant of hoky-poky "deception, fraud," altered from hocus-pocus.
Wiktionary
n. 1 mischievous behaviour, dishonest or shady activity. 2 sexual intercourse or other sexual activity, especially if illicit. 3 A debaucherous act; a sexual act that is considered inappropriate.
Usage examples of "hanky-panky".
After all, the authorities can always do as the Scottish Lord of Justiciary did in the Weinstein case: bring in a watchdog operant as an amicus curiae to be on the lookout for mental hanky-panky.
A certified check is much harder to play hanky-panky with than is a personal check, but Belle was a clever gal.