The original meaning of the word peg is a small stake driven to the ground or onto a wall to hold something down, or an item used as mark.
The word is of Dutch and Germanic origin.
mid-15c., from Middle Dutch pegge "peg," a common Low German word (Low German pigge "peg," German Pegel "gauge rod, watermark," Middle Dutch pegel "little knob used as a mark," Dutch peil "gauge, watermark, standard"), of uncertain origin; perhaps from PIE *bak- "staff used as support" (see bacillus). To be a square peg in a round hole "be inappropriate for one's situation" is attested from 1836; to take someone down a peg is from 1580s, but the original literal sense is uncertain (most of the likely candidates are not attested until centuries later). Peg leg "wooden leg" attested from 1765.
Therefore, to ask the question
What is the origin of "peg down your expenditure"
would be akin to asking the difficult to answer question
What is the origin of "driving me crazy".
English is full of idiomatic usage, like any language.
The world of computer science itself has frequently borrowed and adapted such idiomatic usage.
- Hard drive, floppy drive, flash drive
- software, firmware, vapourware
The commercial world first used the word
- the Renminbi was pegged to the US dollar
- the price of apples was pegged to $0.50 a pound
It is then by no accident that computer professionals would spontaneously say
- the CPU performance was pegged
- the CPU was pegged
"fasten with or as if on a peg," 1590s, from peg (n.). Slang sense of "identify, classify" first recorded 1920. Related: Pegged; pegging.