A disgruntled employee is one who grunts (grumbles) a lot, with an amplifying "dis-" prefix (like "disturb") and frequentative "-le" suffix (like "sparkle").

From the fun book "Etymologicon" by Mark Forsyth:

If a gem frequently sparks, we say that it sparkles. If a burning log frequently emits cracking noises, then it crackles. That’s because –le is a frequentative suffix. With this in mind, let’s turn to grunting. To gruntle is to grunt often. If a pig makes one noise it has grunted, if it grunts again you may add the frequentative suffix and call the pig a gruntler. A medieval travel writer called Sir John Mandeville described the men who live in the desert near the Garden of Eden thus: In that desert are many wild men, that are hideous to look on; for they are horned, and they speak not, but gruntle, as swines do. But the dis- in disgruntled is not a negative prefix but an intensive one. If the verb already carries negative connotations (and something that makes you keep grunting is probably no good), then the negative dis just emphasises how bad it is. Disgruntled therefore means almost the same thing as gruntled.

Happy Monday!