sit on a tack

change of tack

A reversal of an opinion.

hold tack

To last or hold out.

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 for

To substitute; to take somebody's place.

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 one's ass

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