Idioms are usually defined in semantic terms as non-compositional units whose meaning does not result from the meaning of their parts. This definition implies that idioms have exactly one meaning. But if we examine idioms in context in large electronic corpora, we observe that the meaning of idioms is contextually variable, like that of words. We observe uses that exceed the range of the conventionalized and (lexicographically) codified meaning. A case study (the German idiom ins Gras beißen, literally "to bite into the grass", 'to die, to bite the dust') is used as an illustration of the relation between core meaning and contextual variability or meaning extension in idioms and the underlying mechanisms of this process.