Acroches-bas literally translates to "stocking fasteners" or "garters". It's a somewhat archaic and specific term for an item of clothing, making it less common in everyday French and thus more unique to discover in a song.
In the context of "Nazi Rock," Serge Gainsbourg uses this word to evoke a particular, slightly provocative image, fitting the song's controversial and theatrical tone. It adds to the song's distinct atmosphere, inviting listeners to explore its unique vocabulary and themes.
‘Nazi Rock’ opens Serge Gainsbourg's 1975 concept album Rock Around the Bunker, and it names its target in the first line: ‘Voici venir la nuit des longs couteaux’ (‘Here comes the night of the long knives’), a real historical purge turned into a party announcement. From there the song dresses SS men in glam-rock drag: black stockings, garter belts, corsets, red lipstick. ‘Enfilez vos bas noirs, les gars’ (‘Put on your black stockings, guys’) sets the tone for the whole song, Nazi power dressed up as something silly and small.
Gainsbourg wore the yellow star as a child under the Occupation, and the whole album is built on this kind of confrontation, mocking the imagery instead of solemnizing it. The chorus repeats ‘On va danser le’ (‘We're going to dance the...’) until the title finishes the sentence for you: the Nazi Rock, a dance craze built from history's worst men, played for laughs and made to look ridiculous.