Green’s Dictionary of Slang

weasel n.

1. [mid-17C; 20C+] (also weasle) a general derog. term; also as adj. [reverse anthropomorphism; Williams has 17C use of weasel, a lecher].

2. in attrib. use of sense 1.

3. [mid-19C+] (US) a native of South Carolina [the state has a large population of the animal].

4. [1920s+] (US) an informer.

5. [1920s+] (orig. US) the penis [it ‘burrows’].

6. [1930s] (US Und.) a private detective.

7. [1940s+] (N.Z.) a sly or devious person.

In derivatives

weasel-assed (adj.)

a general derog. description.

weasely (adj.) (also weasily)

a general derog. description; image is of untrustworthiness.

In phrases

grease the weasel (v.)

[1990s+] (US teen) to have sexual intercourse.