Green’s Dictionary of Slang

swellhead n.2

also swell bean, swelled-head
[backform. f. swell-headed adj.]
[mid-19C+]

1. a braggart, a boaster, a show-off.

2. in attrib. use of sense 1.

3. (also swelled bean, swelled head, swelled nut, swollen head) conceit, arrogance; thus have the swell-head v. [bean n.1 (4)/nut n.1 (1b)].