Iyokwim is an acronym, meaning a word formed by the first letters of a phrase, in this case standing for "If You Only Knew What I Meant". Specifically used when a double entendre is implied between 2 people within a larger group setting unbeknownst to the rest of the group.
Additionally, 'If You Only Knew What I Meant' can be whistled to the tune from the Wizard of Oz, 'If I only had a brain.' Related to 'If you know what I mean.'
The setting is at a ballpark with a group of friends. One person is going to the concessions for a hotdog and asks, 'Does anyone else want a hot dog?, IYOKWIM?' Another person responds, 'I like mine covered in chili, IYOKWIM'. The rest of the group is confused, but the 2 having just IYOKWIM'd the group are laughing. IYOKWIM.