Merriam-Webster's Collegiate® Dictionary, 11th Edition
PLUMB
I. noun
Etymology: Middle English, from Anglo-French plum, plomb, from Latin plumbum lead
Date: 14th century
a lead weight attached to a line and used to indicate a vertical direction,
II. adverb
Date: 15th century
straight down or up ; vertically, to a complete degree ; absolutely , in a direct manner ; exactly ,
III. verb
Date: 15th century
transitive verb to weight with lead,
2. to measure the depth of with a plumb, to examine minutely and critically , to adjust or test by a plumb line, to seal with lead, to supply with or install as plumbing, intransitive verb to work as a plumber,
IV. adjective
Date: 15th century
exactly vertical or true, thorough , complete , see: vertical