imperceptibly

US /ˌɪm.pɚˈsep.tə.bli/
UK /ˌɪm.pɚˈsep.tə.bli/
1.

อย่างไม่น่าเชื่อ

in a very slight way that a person is unable to notice or feel:

:
•
Gradually, almost imperceptibly, her condition had worsened.