'sign on' definitions:

Definition of 'sign on'

From: WordNet
verb
Engage by written agreement; "They signed two new pitchers for the next season" [syn: sign, contract, sign on, sign up]

Definition of 'sign on'

From: GCIDE
  • sign on \sign` on"\ (s[imac]n` [o^]n"), v. i. To begin a communication, especially one conducted by radio waves. [PJC] Signor

Synonyms of 'sign on'

From: Moby Thesaurus