simple past tense and past participle of sign on

(transitive) To incorporate (something extra, often an amount of money) into something else (often a bill). / (intransitive) To put an expansion onto a building, usually a home.

Alternative form of add-on

To act decisively on the basis of information received or deduced. / Το act accordingly to a (usually sublimated) desire or impulse. / To take action against something. / (medicine) To affect something.

(idiomatic, US, dated) Beyond one's power or ability.

To attend a private meeting or class without participating.

(idiomatic) shove it up your ass (up yours) or back off. / Used other than figuratively or idiomatically: see sit on, sit, on, it.

(idiomatic) To stop people from enjoying an activity.

(idiomatic) Beyond one's level of authority. / (idiomatic) Beyond one's capability.

(idiomatic) To grow in confidence in a new situation as one gains experience.