Hump, Rump, Lump, Bump

Solution 1:

This is a rime; that is, it's the vowel nucleus plus coda of a monosyllable. And many rimes have phonosemantic coherence. As do assonances, that is, initial consonant clusters.

This particular one has a 3-Dimensional sense — an "ump" is something with three dimensions, roughly the same size in all three. There are 15 such simplex words in English:

lump clump dump plump hump slump jump rump
bump crumple stump rumple sump tump mump

Some of the assonances in these words are also phonosemantically coherent. KL- means 'Together', PL- means '2-Dimensional Thick', and ST- means '1-Dimensional Vertical Rigid'. So plump means an ump with wide padding, clump means an ump formed by putting things together, and — my favorite — stump means an ump that used to be a long vertical rigid thing.

See here for much, much more about phonosemantics.

Solution 2:

I don't know whether these words have the same root but it's not an isolated phenomenon - think of how many 'wet' words have sp or sh

splish, splash, sploosh, gush, splatter, spatter, spray, spurt, shower, spew, sperm, spume, squirt, sprinkle

I don't know if these have etymological connections either and I'm afraid it's too late at night for me to start looking them up!

P.S. tump also means 'hillock' or 'mound' and there is a Welsh word twmpath - this from Wikipedia:

Twmpath is a Welsh word literally meaning a hump or tump, once applied to the mound or village green upon which the musicians sat and played for the community to dance. Twmpath dawns were organised by Urdd Gobaith Cymru in the late 1950s and 1960s, a form of barn dance, for the entertainment of young people, mainly from rural areas.[1] These events remained popular until the rise of discos in the 1970s. Twmpath is used today to mean a Welsh version of the barn dance or céilidh.

Solution 3:

This is not a case of fortuitous convergence. Virtually every word with "ump" in it has a meaning that is harsh, derogatory or negative (in varying degrees). Plumb, Thump, Trump, Dump/Dumpster, Chump, Clump, Crumple, Bump, Frump, Grump, Mumps, Lump, Stump, Sump, Slump, Rump, Strumpet. It is no wonder the villains at the baseball park and elsewhere are called "Umpires".

Even softer "ump" words have a feeling about them that is disquieting or intrusive on a well-ordered day (e.g)(Jump, Hump, Pump, Trumpet, Rumple). There is not a lullaby among them and most fairly cry out for attention.