“spread yourself.” Definition from HALOT and Holladay Hebrew-English lexicons. The Brown-Driver-Briggs lexicon was more opaque, only daring to use the Latin: in sensu obscoeno (in an obscene sense). God is being very graphic and not pulling any punches in letting the people of Judah know exactly how he felt about their idolatry. They were in a marriage covenant with Him and they were committing adultery with other gods.
“prostituting yourself..” Ritual sex was often part of the worship of pagan gods. The people of Israel committed spiritual adultery and physical adultery right along with it.