i-adjective
difficult to solve; intractable
Used for problems, puzzles, or situations that are hard to resolve or unravel. Often implies a complex or stubborn nature.
この問題は解き難い。
This problem is difficult to solve.
解き難い謎に挑む。
Take on an intractable mystery.
Compound of the verb stem 解き (from 解く, 'to solve') and the suffix 難い (がたい, 'difficult to do'). The suffix がたい attaches to the continuative form of verbs to express that an action is hard to perform, often with a nuance of inherent difficulty.