With a furrowed or prominent brow, or worried expression. Formerly, with large or bushy eyebrows.
This expression is very old - first recorded in William Langland's The vision of William concerning Piers Plowman, 1362:
"He was bitel-brouwed..."
What was meant by the term at that date is a matter of conjecture. In the 14th century there were two meanings of the noun beetle, or bitel as it was then spelled: either the insect, or a heavy-headed wooden implement used for driving in pegs etc, that is, a mallet.
Our current usage of beetle-browed tends to refer to the forehead, which would suggest the mallet origin. In Langland's day brow always meant eyebrow. Beetle-browed referred to tufted eyebrows, which resembled the antennae of beetles. Many of the early citations refer explicitly to eyebrows; for example, Randle Cotgrave's A dictionarie of the French and English tongues, 1611:
"Beetle-browed - Sourcilleux, having very great eye brows, frowning, or looking sowrely; surlie or proud of countenance."
The weight of evidence points toward the derivation being the resemblance of bushy eyebrows to beetles' antennae.