And Joseph remembered (i.e. the sight of his brethren prostrating themselves before him recalled to his mind) the dreams which he dreamed (or had dreamed) of them (vide Genesis 37:5) and said unto them, Ye are spies (literally, ye are spying, or going about, so as to find out, the verb רָגַל signifying to move the feet); to see the nakedness of the land—not its present impoverishment from the famine (Murphy), but is unprotected and unfortified state (Keil).
Cf. urbs nuda praesidio (Cic; 'Att.,' 7.13); taurus nudatus defensoribus (Caes; 'Bell. Gall.,' 2.6); τεῖχος ἐυμνώθη (Homer, 'Iliad,' 12:399)—ye are come. The Egyptians were characteristically distrustful of strangers,—AEgyptii prae aliis gentibus diffi-dere solebant peregrinis (Rosenmüller),—whom they prevented, when possible, from penetrating into the interior of their country.
In particular Joseph's suspicion of his Canaanitish brethren was perfectly natural, since Egypt was peculiarly open to attacks from Palestine (Herodotus, 3.5).