Ad Filum Aquæ
Defined in 2 dictionaries — Black's (1910), Black's (1891)
Black's Law Dictionary
Henry Campbell Black, M.A. · 1910
To the thread of the water; to the central line or middle of the stream. Usque ad fllum aquæ, as far as the thread of the stream. Bract, fol. 208b; 235a. A phrase of frequent occurrence in modern law; of which ad medium fllum a quæ (q. v.) is another form.
A Dictionary of Law
Henry Campbell Black · 1891
To the thread of the water; to the central line, or middle of the stream. Usgque ad filum aque, as far as the thread of the stream. Bract. fol. 2086; 235a. A phrase of frequent occurrence in modern law; of which ad medium filum ague (gq. v.) is another form.