Possible Duplicate:
Difference between phrase and idiom
Is there a name for phrases which without context (cultural, historic, etc.) would not be understandable. Such as "This is not my strong suit". I thought at first it is called an aphorism but after google I am not so certain anymore.
Merriam-Webster says:
aphorism, noun :
2 : a terse and often ingenious formulation of a truth or sentiment usually in a single sentence : ADAGE, MAXIM
"Not my strong suit" is more of an idiom:
idiom, noun: 2 : an expression established in the usage of a language that is peculiar to itself either in grammatical construction or in having a meaning that cannot be derived as a whole from the conjoined meanings of its elements
The Online Etymology Dictionary dates "strong suit" to 1865, and says it came from card-playing, where "suit" refers to one of the four suits (clubs, diamonds, hearts, or spades). Someone who played cards would probably consider "strong suit" more of a metaphor.
A phrase whose literal meaning differs from its figurative one is an idiom and such usage is idiomatic, such as
cooking the books
referring to creative or fraudulent financial accounting.
An aphorism is more of a cleverly phrased observation or truth:
better late than never
Everybody's language experience is different, so That's not my strong suit would be a transparent metaphor from a card game like bridge or whist to anyone familiar with those games, calculably understandable by others from the sense of strong alone, and probably incomprehensible to still others. There is no single word that describes all of these attributes, sorry; in fact, that's a lot to expect of one word.
Since I learned bridge in college 50 years ago, all my memories of the use of strong suit are bridge-colored, and it makes perfect sense as a metaphor. Indeed, a familiarity with the actual source of a metaphor -- in this case, table talk, and the strategies and tactics that it typically accompanies -- can make for a more nuanced understanding on occasion, I think.
As to the more general question, practically every expression is hard to understand, outside its actual context. People leave off things that they expect everyone to know. And they're often wrong, especially in writing, where one frequently knows nothing of one's readers.
Idiom is phrasing that is peculiar to a language, i.e. language as it is actually used. This requires knowledge of the culture (idiom differs regionally) but not necessarily knowledge of history. Expressions like the one in your example are typically idiomatic.
