means Covered in or full of small, irregular bumps or clumps, so the surface or substance feels uneven rather than smooth.

from Built from 'lump' plus the adjective ending '-y,' the same little suffix that turns 'mud' into 'muddy.' 'Lump' itself is a medieval English word of obscure origin — probably native and onomatopoeic, the sort of thick, blunt sound that suits a shapeless mass — with possible cousins in Scandinavian and Low German words for clumps and chunks. 'Lumpy' as the bumpy adjective settled into common use by the 1700s.