Literally, penis. There is no standardised spelling of this term since it's a transiteration from the original Hokkien phrase. 'Cheow' means 'bird', which alludes to the English usage of the word 'cock' to refer to the male member.
Lancheow however is used in a wider literary sense in Singlish to describe a broad category of unpleasant things, including asinine people, as well as as an expletive exclamation.
Wah ... your lanchoew damn big. (Your penis is big.)
Lanchoew lah! Kenah arrowed again. (Used as an expletive exclamation)
That stoopid asshole, sibeh lancheow one ... (Used as an adjective)
This fuck is simi lancheow? (What the fuck is this?)