Meaning of the phrase "Four pounds if he's an ounce" [duplicate]
Solution 1:
There's an ellipsis at the beginning of the sentence (John Lawler tells us that this is conversational deletion):
[He is] four pounds if he’s an ounce.
Otherwise this is a perfectly ordinary conditional, described this way in Chapter 10, ‘Rhetorical Conditionals’, of Renaat Declerck and Susan Reed, Conditionals: A Comprehensive Empirical Analysis, 2001, p.345. (Their terminology rests on the stock formula for conditionals, ‘If P, then Q’).
In such examples (in which both clauses typically use an indication measure, amount, number, etc.), the function of the if-clause is to emphasize that there is no doubt whatever that the Q-clause is true. This type has much in common with a direct inferential, since it expresses ‘P is patently true, hence Q must be true too.’ However, the if-clause does not express a real premise, which (among other things) is clear from the fact that it typically follows the Q-clause (whereas premise-expressing P-clauses seldom do). Rather than being the P-clause of an inferential […] the if-clause is a purely rhetorical device to emphasize the truth of the Q-clause.
The fish obviously qualifies as weighing at least an ounce; argal, my assertion that he weighs four pounds is clearly true.
Solution 2:
The purpose of this idiomatic logic bomb is to establish a fervent belief in the truth of the statement at hand.
~~~The unreality of the falsity emphasizes and gives validity to the subjective "truth" that you are trying to put across in your statement.~~~
For example, if we went fishing and I stated, "Hey, Nice catch! Looks like it is at least three pounds."
You might respond, "Three pounds? Look at this big fellow; he is four pounds if he is an ounce."
You made the statement to emphasize the surety of your estimate.
This "comparison" between a false thing and a true thing uses the falsity to emphasize the truth.
"That man is sixty years old if he's a day."
Similar uses can also be done to emphasize a falsity:
"If she's a virgin, then I am a faery princess."
Hence, the unreality of the falsity emphasizes and gives validity to the subjective "truth" that you are trying to put across in your statement.