Rashi on Numbers 14:44/רש"י על במדבר י״ד:מ״ד

1ויעפלו is an expression for “strength” (doing a thing by insolent force). Similar is, (Habakkuk 2:4) “Behold it is insolent (עפלה)”, engres in O. F., an expression for insolent force. Similar also is, (Micah 4:8), “stronghold (עפל) of the daughter of Zion”; (Jes. 32:14) “the forts (עפל) and towers”. The Midrash Tanchuma 4:4:19, however, explains it (ויעפלו) to be of the same meaning as אופל, darkness, i.e. they went in darkness — without permission.