1901 A. Hodder lii. 413 Serious-mindedly,—being snow-bound should make every one serious-minded,—the importance you give..to eloquence..is [etc.].
1966 ‘W. Cooper’ i. v. 64 I heard two people serious-mindedly handing them to and fro.
2007 (Nexis) 31 Mar. 6 Now I would rather talk very serious-mindedly about something..than have a jokey conversation.