Syllable division derived from spelling — this word is not in the pronouncing dictionary, so no phonetic transcription is shown rather than a guessed one.
whatchamacallum is a noun meaning "something unspecified whose name is either forgotten or not known". It carries one recorded sense.
something unspecified whose name is either forgotten or not known
“she eased the ball-shaped doodad back into its socket”
“there may be some great new gizmo around the corner that you will want to use”
Forms marked · are formed by the regular rule rather than recorded in the source data. English keeps a few hundred irregulars and the rest follow the pattern, so the rule is right far more often than not — but it is a rule, and the mark says so.
Definitions, synonyms and semantic relations for whatchamacallum come from Princeton University’s WordNet. Commonness is measured against a corpus of everyday English. Full sources and licences.