Late 18th century (in the sense 'perpendicularity, steadiness'): from French, from à plomb 'according to a plumb line'.

• plumb from Middle English:

  You can say that something which is not quite perpendicular is out of plumb. This draws on the original meaning of plumb, a ball of lead attached to a string to determine a vertical line, or a plumb line. Another early use was as a term for a sounding lead used for measuring the depth of water. To plumb a body of water was to measure its depth in this way, and is the source of the phrase plumb the depths. The source of plumb is Latin plumbum ‘lead’, also the root of plumber. Medieval plumbers dealt in and worked with lead, and it was not until the 19th century that the word was applied solely to people trained in fitting and repairing water pipes, which were initially all made of lead. The Latin word plumbum is also the basis of plummet, which came into medieval English from Old French and then referred to a plumb line. The use of plummet as a verb meaning ‘to drop straight down rapidly, to plunge’ is more recent, first recorded in the 1850s. An early use of the verb was ‘to let a vertical line fall by means of a plummet’, and the modern sense developed from this. To do something with aplomb (late 18th century) comes from the French phrase à plomb, ‘straight as a plumb line’. Plunge (Late Middle English) also comes from plumbum, this time via Old French plungier ‘to thrust down’. The phrase take the plunge dates from the mid 19th century.