sit in for

sit back

To relax, to not exert oneself.

sit in

To attend a class, educational presentation, meeting, etc. for one time, as a visitor.

sit in with

To join (often as an observer, supervisor or outsider) a group working together on a task.

sit on

To block, suppress, restrain (something).

sit on a tack

An expression of contempt or dismissiveness directed at someone, particularly in the case of disagreement.

sit on one's ass

To be very lazy and idle, especially when immediate action is required.

sit on one's hands

To remain idle when action is needed, perhaps because of fear, ignorance, complacency, indecisiveness, or dislike of the person(s) or situation.

sit on the fence

To remain neutral on a certain topic, to not have a stance or opinion.