سَقْى

in the phrase سَقْىُ الفُرَاتِ, which means The towns, or villages, [or lands,] watered by the Euphrates, is said by Mtr to be a verbal noun used as a subst. [properly so termed, and, being originally a verbal noun, it may be used alike as singular and plural]; or, in this phrase, a noun that should be prefixed to it [such as ذَات], is suppressed: or, according to some, it is سِقْى [q. v.], an instance of the measure فِعْلٌ in the sense of the measure مَفْعُولٌ; and thus it is in the handwriting of EI- Hareeree in his 22nd Makámeh. (Har p. 246.)

See also the next paragraph.