Fine-Force

Fr. [fine, from Fr. fin, crafty or subtle; or artificial or exact.] In old English law. An absolute necessity or constraint, not avoidable. When a man was constrained to do that which he could in no way avoid, he was said to do it de fine force.

Cowell.

Old N. Brev. fol.

78. Stat. 35 Hen. VIII. c. 12.

So a fine force, (q. v.) The word fine seems to be used in this phrase, in the same sense as the modern word pure.