What is the meaning of If_only?

Used other than figuratively or idiomatically: see if, only.

I wish that; signifies a wish or desire for the present or the future; would that; would (both chiefly archaic).

I wish that; signifies a regret about an action in the past; would that; would (both chiefly archaic).

Even if for no other reason than; only just.

Signifies a hypothetical or wishful precondition, or one which would be simple but is nonetheless unlikely, for a hypothetical, counterfactual, or future outcome.

Source: wiktionary.org