[mohr-SO] (n. m.) A strange case of shifting spellings, or not in English's case. Going way back, the Latin morsellum means "a small piece" and is made up of morsum (piece) and the diminutive -ellus. To get horribly specific about what kind of piece this is, it's good to know the word is derived from mordeo, or "to bite." Right away, the French had the good sense to ablate the word from morsellum to morsel, which is where the English left things.
During the Middle Ages, the French went through a period of systematically (that is, with a semblance of consistency) altering the spelling of Latin words they borrowed. One of these so-called Latin rules as they are called in Auguste Brachet's 1868 Dictionnaire Etymologique is that the s before a vowel often became a c. Hence, the Latin salsa became sauce and morsel begat morcel. Another Latin rule was that long vowel sounds should be shortened and the natural way of doing so was to slide down the vowel ladder which, both phonetically and alphabetically, goes a, e, i, o, u. (Brachet even points out that a starts at the base of the larynx while u expires on the lips, a natural order.) So the -el sounds often become an o sound and agnellus became agneau (lamb) and morcel became morceau.