Unless your making pickles, cucumbers are not crucial so this is a saying when you declare something as not necessary.
As Mary was getting ready for her job interview, she realized that she had neglected to brush up on her Spanish. As this job required no language other than French, she calmed herself and realized her concerns were as crucial as cucumbers.
Pretty simple really, a combination of the truth of reality and something that is crucial to your well-being. In other words, it is something, you've "gotta have". In all honesty, this word originated on 3 Reinert at Saint Louis University...