noun-modifying (irregular)
intricate; complicated; complex
Describes situations, stories, relationships, or problems that are tangled, involved, or hard to unravel. Often used in the prenominal form 込み入った.
込み入った事情があって、詳しくは話せない。
There are complicated circumstances, so I can't go into detail.
この小説は込み入った人間関係を描いている。
This novel depicts intricate human relationships.
複雑 is a general adjective/noun for 'complex' or 'complicated', while 込み入った specifically suggests something tangled or involved, often with a nuance of being hard to sort out.
Derived from the verb 込む (to be crowded, to go into) combined with 入る (to enter), forming 込み入る (to become complicated or tangled). The prenominal form 込み入った is used as an adjective.