Verb: An attempt to masturbate while walking. From the clumsy combination of the words 'wank' + 'walk', and not managing either to any degree of success.
Noun: The onomatopoeic sound indicative of a failure to multitask, particularly experienced by the male of the species. Sometimes confused with 'thwack' as it usually occurs following a fall or a collision with some object.
"What was that sound?"
"It's my flatmate bumping into furniture again; he thinks wlaking will improve his ability to juggle at the clown convention next week."