1 weight, (plumb-)bob, plummet, lead, sinker:
The plumb holds the string taut and exactly vertical.
2 vertical, perpendicular, straight up and down:
Make sure the jamb is plumb before hanging the door.
3 vertically, perpendicularly, straight up and down:
The cord doesn't hang plumb because the shelf is in the way.
4 exactly, precisely, dead, right, accurately, Colloq slap, Brit bang, spot:
The navigator checked our position and we were plumb on course.
5 sound, fathom, measure, probe, explore, measure, gauge, delve, penetrate:
The bathyscaphe has plumbed the deeps of the Indian Ocean. I plumb the depths of depression thinking of starving people everywhere.