IPA: /ˈsɪŋɡli/
KK: /ˈsɪŋɡli/
In a way that involves only one person or thing; separately or individually.
She completed the project singly, without any assistance from her team.
Singly → It is formed from "sing" (from Old English "singan", meaning to sing) and the suffix "-ly" (meaning in a manner of). The word "singly" means in a manner of singing or alone, indicating doing something individually.
Think of someone 'singing' ('sing') alone or 'individually' ('-ly') — that's why singly means to do something by oneself.