1 [always + adverb/preposition]
to appear as a large unclear shape, especially in a threatening way
loom up/out/ahead etc
Suddenly a mountain loomed up in front of them.
if a problem or difficulty looms, it is likely to happen very soon:
An economic crisis is looming on the horizon.
to seem important, worrying, and difficult to avoid:
Fear of failure loomed large in his mind.