"Get lost" is "滚开" or "滚蛋" but it is not an idiom, just a slang
If you want an idiom that contains "fart" and meaning of "go away", “屁滾尿流” is the only possible one.
“屁滾尿流” means “farting and pissing uncontrollably (mostly while running away)” It is a metaphor for “in extreme fear”
殺得敵人屁滾尿流地逃走 (beating the enemy so bad that they ...
This thread would never be complete without the "urban-dictionary" style (i.e, slang), modern example for such idiom, the good old 吹牛, or its R-rated version 吹牛逼.
It generally means to brag about something, which could be true but usually isn't.
But you can coin four-character words to express it more accurately. It looks like that you mean academic claims.
Let me have a try:
论訇证寡（The claim is thunderous(figurative), but the evidence is little.）
Note: Only in writing you can coin such words. Otherwise people won't understand you.