This is a fairly common 'cautionary' final verse in 19th or early 20th century songs. The 'Dundee Weaver', mentioned by Jim McLean, is typical.
Come a' ye Dundee weavers now tak this advice fae me,
Never let a fellah an inch abane your knee,
Never stand at the back o' a close or up agin the wa'
For if ye dae ye can safely say your thingummyjig's awa',
For if ye dae ye can safely say your thingummyjig's awa'.
As a result of intensive personal research I can assure Mudcatters that, despite the reinforcement of the repeated last line, few young Dundee ladies of my acquaintance, regardless of occupation, paid the slightest heed to that particular caveat. Bless them!