also: するがよい
expression, i-adjective (yoi/ii class)
had better ...; may as well ...
A somewhat old-fashioned or literary expression used to give advice or permission, often with a tone of resignation or suggestion. Attaches to the dictionary form of a verb. In modern casual speech, ～したほうがいい or ～すればいい are more common.
必要なら、今すぐ電話するがいい。
If it is necessary, you may as well call right now.
好きにするがいい。
Do as you like.
More common and neutral way to say 'had better' in modern Japanese.
Common expression for 'should do' or 'may as well do', less literary than するがいい.
Combination of the verb する (to do) in its attributive form, the particle が, and the adjective いい/よい (good). The construction literally means 'doing is good', which developed into a suggestion or mild command.