you can't be just a little bit pregnant

It means that the situation that is being described is a binary. It is something that is either IS or ISN'T. Such as is the case when one is pregnant. One either "is" pregnant or "is not", there is no in between or partial pregnancy. It means it is a situation in which there is no ambiguity or no gray area. You can't be "just a little bit" of certain things.

You can't be just a little bit pregnant when you are talking bankruptcy. You can't be just a little bit bankrupt. You are either bankrupt or not bankrupt.

References:
Lou Mannheim (Hal Holbrook) in the movie "Wall Street" used this phrase while conversing with Bud Fox

51👍 6👎