1. [mid-19C–1910s] to watch, to survey, to notice.
2. [late 19C+] to inform to the police.
3. [late 19C+] to betray someone; to inform upon.
4. [late 19C+] (mainly Aus./N.Z.) to annoy, to irritate.
5. [1930s+] to complain, to nag.
[late 19C] an arrest made on the evidence of an informer.
[1930s] (Aus. Und.) to betray a plan.