Wiktionary
undefinably
adv. In an undefinable manner
Usage examples of "undefinably".
There had been an odd undernote in that assurance that, undefinably, he did not like.
No wonder there had been something naggingly but undefinably different about Pinder, which he hadn't been able to put his finger on in two days.
There was a moment during which I listened, reminded of the faint sense I had had, the first night, of there being something undefinably astir in the house, and noted the soft breath of the open casement just move the half-drawn blind.