Green’s Dictionary of Slang

blow one’s top v.

also blow one’s cap, ...topper
[SE blow, explode + top n. (3)/SE cap/topper n.3 (1); the image is of a volcano]
(orig. US)

1. [1920s+] to lose one’s sanity.

2. [1930s] (US black) to get drunk.

3. [1930s] (US tramp) to commit suicide, esp. by shooting.

4. [1930s+] (also blow gaff) to lose one’s temper, to become violent [gaff n.2 (4)].

5. [1930s+] to express intense emotion; to become very excited.

6. [1940s–50s] to talk too much.