groundbass [graʊnd beɪs] n.
1.) Mus. A bass-passage of four or eight bars in length, constantly repeated with a varied melody and harmony.
2.) Fig. An undercurrent (Oxford English Dictionary 2nd Edition).
Etymology: ground, from Middle English, from Old English grund + bass, from Middle English bas, base, now spelled bass after Italian basso, but still pronounced as base. (Alexander Pope rimed base and ass.)
"An intelligent director can and should ensure that the on-stage audience demonstrates some awareness of the groundbass of mortality sounding underneath the hilarity generated by Bottom's performance, that a line like Lysander's "he is dead, he is nothing" (V.i.308-9) is not lost in the merriment" ("General Introduction" to The Riverside Shakespeare, Anne Barton, 1974).