Wiktionary
what do i know
phr. (context idiomatic English) Implies that a statement is based on a guess or assumption rather than on knowledge or evidence.
Wikipedia
What Do I Know
"What Do I Know" is the title of a debut song recorded by American country music group Ricochet. It was released in December 1995 as the first single from their self-titled debut album. The song reached #5 on the Billboard Hot Country Singles & Tracks chart in April 1996. It was written by Stephony Smith, Sunny Russ and Cathy Majeski.
Usage examples of "what do i know".
And what do I know of him, this great Emperor, this Dragon of Roma, this distant ancestor of mine?
On the other hand, the meal he served was better than I would expect from a miser, so what do I know?