to have someone by the short and curlies

[rupertbrooke]

Very informal and vulgar slang.
To have complete power over someone:-

They've got us by the short and curlies. We have no choice but to agree.

Seems to have originally meant 'by the short hairs of the neck' but now almost exclusively of the pubic hair.

 

[cougr]

Μας κρατάνε από τα αχαμνά/τρυφερά(ie groin/tender bits). I'm sure there are other suitable expressions but they don't come to mind right now.