Green’s Dictionary of Slang

shoot off v.

1. [mid-19C+] to leave quickly (cf. shoot v. (2b)).

2. [mid-19C+] to talk, usu. irresponsibly or indiscriminately; most often in phrs. below.

3. [1920s+] to ejaculate [ext. of shoot v. (1a)].

4. [1960s] to reprimand, to threaten.

5. [1960s+] to lose one’s temper.

In phrases

shoot off at the mouth (v.) (also shoot off at the lip)

[late 19C+] (US) to talk irresponsibly.

shoot off one’s face (v.) (also shoot one’s face off, shoot with the face)

[late 19C–1950s] (US) to talk effusively.

shoot off one’s head (v.) (also shoot off one’s pan, shoot one’s head off) [pan n.1 (2)]

1. [1910s–50s] to talk, esp. boastfully.

2. to complain, to make a fuss, to argue.

shoot off one’s jib (v.) (also shoot one’s jib off) [jib n.1 (1)]

[1950s–60s] to talk, esp. arrogantly or boastfully.

shoot off one’s mouth (v.) (also shoot off one’s bazoo, …lip, …yap, shoot one’s mouth off, ...bazoo off, …gums off, …kisser off, shoot one’s mouth (at)) [sense 2 + bazoo n.1 (1)/kisser n. (1a)/yap n.1 (1a)] (orig. US)

1. [mid-19C+] to talk, esp. in a loud or boastful way.

2. [1930s+] (also shoot one’s mouth) to betray secrets.

3. [1960s+] to vomit.

shoot off the fat (v.) [ety. unknown]

[mid-19C] (Can.) to ask for credit.