I had to look this up and Wikipedia says two things that I didn't expect. Looks like "sympathizer" wasn't far off:

• Originally a shortening of "simpleton", the New Partridge Dictionary of Slang and Unconventional English traces usage of the noun simp to 1903.
• Simp began to have the connotation of someone being "soft" and "overly sympathetic" in the 1980s, when it was used by West Coast rappers [...] Simp was referenced by Sir Mix-a-Lot in his 1992 hit "Baby Got Back" in the lyrics, "A lot of simps won't like this song".