hanky-panky
noun phrase (humorous)

- behaviour, in particular romantic or legally dubious behaviour, considered improper but not serious

(Oxford Dictionary)

---
Hanky-panky is a so-called reduplicate, which are expressions that repeat a part of one word, either exactly or with a slight change, to form a phrase.

Many reduplicates, like hanky-panky, are formed with nonsense words that have no relation to the meaning of the phrase itself. For instance, although hanky is short for handkerchief, the word panky does not exist. It simply rhymes with hanky.

Other common reduplicates include:

hocus-pocus = meaningless talk or activity, typically designed to trick someone or conceal the truth of a situation (Some people still think psychology is just a bunch of hocus-pocus)

mishmash = a confused mixture (The new office park is a mishmash of building styles)

pitter-patter = a quick, light knocking sound (The pitter-patter of children's feet could be heard through the house)

---
SYNONYMS

(romance)

affair, dalliance, fling, liaison, romance

(business dealings)

chicanery, deception, funny business, machinations, mischief, monkey business, shenanigans, skulduggery, subterfuge, trickery

---
Practice OWAD in a conversation:

"Unfortunately for Bill Clinton, his reputation was forever damaged by HANKY-PANKY in the White House."