Lumpy means full of small solid bits that should not be there. Lumpy gravy, a lumpy mattress, lumpy custard.
In cooking it is always a fault, and the cause is nearly always flour or powder added to hot liquid too fast — adding a little liquid to the powder first is the fix. Of a mattress or a road it means uneven. Of data it means unevenly distributed, which is a real technical use in demand planning. Note lump; and lump it, meaning to put up with it.
1adjectivelumpyFull of small solid bits that should not be there.
“Keep stirring, or the gravy will go lumpy.”
Patternlumpy + noun (sauce, mattress, porridge) · go lumpy · a bit lumpy · lumps
2adjectiveunevenUneven, of a surface or a distribution.
“The mattress was old and lumpy, and I hardly slept.”
lumpy is a quality of unevenness.
LUMPy → full of LUMPS
It is lump + y: stir the sauce badly and it fills with small hard lumps.
hooks ontolump
Proto-Germanic
*limpaną — to glide, go, hang loosely
Middle English
lumpe
English
lump
English, today
lumpy
From lump + -y. lump comes from Middle English lumpe, from a gem - base akin to Proto-Germanic limpaną (“to glide, go, hang loosely”). Compare Dutch lomp (“rag”), German Low German Lump (“rag”), German Lumpen (“rag”) and Lump (“ragamuffin”).
Etymology from Wiktionary, CC BY-SA