also: のしのし
adverb, adverb taking the 'to' particle
walking heavily; lumbering
Mimetic adverb describing a heavy, plodding gait, often with large steps. Typically used with と and a verb of motion like 歩く.
大きなクマがのっしのっしと歩いている。
A big bear is lumbering along.
彼は疲れた様
He trudged heavily up the stairs, looking exhausted.
どしどし implies a more forceful, stomping sound or rapid, energetic progress, while のっしのっし focuses on a slow, heavy, swaying gait.
Onomatopoeic/mimetic word; the exact derivation is uncertain, but it is likely based on the sound and rhythm of heavy footsteps.