Hanky Panky American old-fashioned noun phrase informal
Refer to slightly improper behavior or sexual activity between couples but it's not too serious
No hanky-panky until you are 18 years old.
Refer to activities that is not good, honest and suspicious
You should stop your economic hanky-panky or you will be investigated and arrested.
A humorous way of saying an inappropriate or unacceptable action that disrupts a party or other social gathering
If you describe someone as shifty-looking, they seem dishonest, untrustworthy, or suspicious.
This is one of those nonsense terms created only to have a catchy allusion or rhyme, like 'the bee's knees', 'the mutt's nuts' etc. The words themselves have no inherent meaning, although it is possible that 'hanky-panky' derives as a variant of 'hoky-poky' or 'hocus-pocus'.
The term is first recorded, in relation to its original 'trickery' meaning, in the first edition of 'Punch, or the London Charivari', Vol 1, September 1841:
"Only a little hanky-panky, my lud. The people likes it; they loves to be cheated before their faces. One, two, three - presto - begone. I'll show your ludship as pretty a trick of putting a piece of money in your eye and taking it out of your elbow, as you ever beheld."
The second meaning refers to sexual activity or dalliance, especially of a surreptitious nature" has been with us since the middle of the 20th century, as here from George Bernard Shaw's play Geneva, 1939:
She: No hanky panky. I am respectable; and I mean to keep respectable.
He: I pledge you my word that my intentions are completely honorable.
(Source: phrase.org.uk)
Used to allude that the last force, problem or burden which is seemingly minor and small causes a person, system or organisation to collapse or fail
Her husband's violent act last night was the straw that broke the donkey's back and she left him