na-adjective
seeming to have a reason; looking like there's more to it
Describes a person, expression, or situation that appears to have some hidden reason or special circumstances behind it. Often used when someone seems to be acting with an undisclosed motive.
彼の仔細ありげな笑みが気になった。
His smile, which seemed to suggest there was more to it, bothered me.
彼女は仔細ありげにうなずいた。
She nodded as if she knew something more.
仔細 is a noun meaning 'reason' or 'circumstances'; 仔細ありげ adds the suffix ありげ ('seeming to have') to describe an appearance of having such reasons.
ありげ is a suffix meaning 'seeming to have' or 'appearing to be'; it attaches to nouns to form adjectival nouns indicating an appearance of possessing that quality.
Compound of 仔細 (shisai, 'reason; circumstances') + ありげ (arige, 'seeming to have'). The suffix ありげ derives from classical あり (ari, 'to be') + げ (ge, 'appearance').