1. Connection to "Plumb Line":
* A plumb line is a weighted string used to determine verticality. It's possible that "plumb forgot" originated from the idea of something sinking so deeply into forgetfulness that it was "plumb" (completely) out of one's memory, as if it had fallen to the bottom of a well.
2. "Plumb" as an Adverb:
* The word "plumb" has been used as an adverb meaning "completely" or "thoroughly" since the 16th century. This usage likely stems from the same concept of verticality associated with the plumb line. Thus, "plumb forgot" would mean "completely forgot".
3. "Plumb" as a Verb:
* The verb "plumb" can also mean "to understand thoroughly." This sense may have contributed to the phrase "plumb forgot," implying a complete and utter forgetting of something.
Regardless of its exact origin, "plumb forgot" is a vivid and expressive phrase that emphasizes the depth of forgetfulness.
It's important to note that the phrase is primarily used in informal contexts and might be considered somewhat regional or dialectal.