The 'country bumpkin' sense plays off
海归 (hǎiguī, 'sea turtle', i.e. someone returned from study abroad). The pun is between
龟 (guī, turtle) for the well-traveled and
鳖 (biē, soft-shell turtle) for the homebody. Use carefully — it can be self-deprecating ('I'm just a
土鳖') or insulting depending on tone.