Synset

making great mental demands; hard to comprehend or solve or believe
<adj.all>   freq. cnt.: 2 freq. cnt.: 1 freq. cnt.: 1

baffling

elusive

knotty

problematic

problematical

tough

sample sentences

"a baffling problem"
"I faced the knotty problem of what to have for breakfast"
"a problematic situation at home"