"Secondary hull" appears in dialogue once in TNG, but doesn't seem to have become a regular part of the jargon until Voyager, where it turns up quite a few times. I think that portion of the ship was mostly referred to as the "Engineering section" in TOS (also in several TNG eps.)
There's also "The Apple", where Kirk tells Scotty "Discard the warp drive nacelles if you have to, and crack out of there with the main section, but get that ship out of there!"
It's unclear whether he means a saucer separation (which would imply the engineering hull was the secondary section), or literally jettisoning just the nacelles (leaving saucer + engineering hull as the "main section"). The former fits better with later Trek
(have we ever seen just the nacelles jettisoned?) and seems to be the general fan assumption, however.