Numbers 14:44

ויעפלו 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.

וַיַּעְפִּ֕לוּ לַעֲל֖וֹת אֶל־רֹ֣אשׁ הָהָ֑ר וַאֲר֤וֹן בְּרִית־יְהֹוָה֙ וּמֹשֶׁ֔ה לֹא־מָ֖שׁוּ מִקֶּ֥רֶב הַֽמַּחֲנֶֽה׃

But they presumed to go up to the hill top: nevertheless the ark of the covenant of the Lord, and Moshe, departed not out of the camp.