The adjectival form describing someone or something introduced secretly or smuggled in. Galatians 2:4 uses it for 'false brothers' secretly brought in to spy on the freedom believers have in Christ. The term suggests infiltration rather than legitimate entry—these aren't genuine converts or honest inquirers but planted agents with hostile intent. The adjective exposes the covert nature of the threat to gospel freedom, warning that not all who claim brotherhood come with pure motives.