Why do I need to know habillé comme l’as de pique?

Because Parisians might not forgive you your lack of fashion sense.  

What does it mean?

Être habillé comme l’as de pique – to be dressed like the ace of spades – is used colloquially to describe someone wearing clothes that, to put it kindly, aren't easy on the eye. 

So, next time you see someone displaying questionable fashion tastes, you might say il/elle est habillé/e comme l’as de pique

Origins

No one really knows where and when the expression originated, but according to the writer Delphine Gaston it comes from uncanny resemblance of the shape of the ace of spades to a hen’s rump.

The spade has traditionally been seen as the weakest colour of the cards in France and is often associated with bad luck or negative outcomes.

Molière, the classic French playwright, used “ace of spades” as a synonym for idiot in his play Le dépit amoureux.

Alternatives

Fagoté comme l’as de pique – an outdated version of habillé comme l’as de pique.

Fagoter means to twirl branches and twigs together until they become a fagot (brushwood). In other words French people used to compare poorly dressed people with shoddy brushwood. 

Être mal sapé – saper is a colloquial word meaning “to be dressed”. Therefore, être mal sapé means “to be badly dressed”