Green’s Dictionary of Slang

mod n.2

[abbr. SE modernist]
[1960s+]

1. a member of a teenage cult orig. c.1961, who wore smart clothes, rode motor scooters and fought their main rivals, the motorcycle-riding, leather-clad ‘rockers’.

2. attrib. use of sense 1.

3. (Aus.) one who discards the conventional past and looks to the future.

4. (US campus) a well-dressed, fashionable person.

In derivatives

moddy (adj.)

[1970s] in a ‘mod’ style.