From some + when.
somewhen (not comparable)
- (rare, nonstandard) at some time; indefinitely; some time or other, sometime
1864, Robert Browning, “Mr. Sludge, "The Medium"”, in Wikisource, line 505[1], retrieved 2012-01-18:Out of the drift of facts, whereby you learn / What some was, somewhere, somewhen, somewhy?
1891, Thomas Hardy, chapter XXXVI, in Tess of the d'Urbervilles - A Pure Woman[2] (Fiction):Yes, though nobody else should reproach me if we should stay together, yet somewhen, years hence, ...
2002, Marc Lange, “What Is Spatiotemporal Locality?”, in An Introduction to the Philosophy of Physics: Locality, Fields, Energy, and Mass[3] (Science / Philosophy), Wiley-Blackwell, →ISBN, page 4:An event is a particular thing that happens somewhere and somewhen.
2008, Michael F. Wagner, “Chapter One”, in The Enigmatic Reality of Time[4] (Science / Physics), Brill, →ISBN, page 26:… things always exist or occur somewhen as well. They exist or occur whenever — at whatever times-when — they in fact exist or occur, and not somewhen else.
2009, Marta Randall, “Secret Rider”, in Robert Silverberg, editor, Trips in Time: Time Travel Tales (Collection of Short Stories)[5] (SciFi), Wildside Press, →ISBN, page 120:Somewhen, curling through the intricacies of tau, John/Johan still lived. Somewhen on this very planet he lived, …
at some time; indefinitely; some time or other, sometime
- Century Dictionary and Cyclopedia, somewhen