Why was the term (perfect) originally attached to forward secrecy
No reason is given by Christoph G. Günther in An Identity-Based Key-Exchange Protocol (in proceedings of Eurocrypt 1989), which is the earliest use on record and claims the notion:
This modification restores a property of the Diffie-Hellman scheme, which we could call perfect forward ...
Forward secrecy of key-establishment (KE) protocol means, that compromising of long-term keys of parties doesn't affect secrecy of established keys in the past.
Example to clarify what PFS means.
Say parties A and B have private keys $s_B$ and $s_B$.
Today they run KE and establish a key $k_1$.
They use this key during a session, and then destroy this key.