1
general::
Phrase(s): sit tight
to wait; to wait patiently. (This does not necessarily refer to sitting.) • Just relax and sit tight. I’ll be right with you. • We were waiting in line for the gates to open when someone came out and told us to sit tight because it wouldn’t be much longer before we could go in.
McGrawhill's American Idioms And Phrasal Verbs
2
general::
Be patient, take no action, as in If you just sit tight I'm sure your passport will be returned to you. [Colloquial; first half of 1700s]
American Heritage Idioms