From raqa'; beaten out, i.e. A (metallic) plate -- broad.
see HEBREW raqa'
H7555. riqqua
[רִקּוּעַ] noun [masculine] expansion; — plural construct פַחִים רִקֻּעֵי אֹתָם וְעָשׂוּ Numbers 17:3 expansions of plates, i.e. they shall be beaten into broad plates.
I. רקק (√of following; Arabic be thin, also figurative be weak, slender, scanty, etc.; Ethiopic be thin, thin; Syriac Pa`el, Aph`el make thin, thin).