Green’s Dictionary of Slang

1. (orig. US) a cowardly, obsequious person, a toady, one who curries favour.

1849		Yale Banger 6 Nov. in Hall (1856) 34: When Boot-lick hypocrites upraised their might.
1850		Yale Battery 14 Feb. in Hall (1856) 35: Then he arose, and offered himself as a ‘boot lick’, to the Faculty.
1900		E.H. Babbitt ‘College Words and Phrases’ in DN II:i 24: boot-lick, n. One who tries to curry favour.
1989		H. Rawson Dict. of Invective (1991) 57: The short form, boot-lick, may be used either as a noun, referring to the person, or as a verb.

2. an act of toadying, obsequiousness.

1900

Virginia Pilot (Norfolk, VA) 25 Mar. 4/3: The cringing and obsequious diplomacy of the bootlick.