Just In Case Meaning

(idiomatic) In the event; should there be a need.

Example:   I'll take an umbrella, just in case (it rains).‎
1629, Roger Williams et al., The Correspondence of Roger Williams, Brown University Press (reprinted 1988, ISBN 0874513863), page 204,
  Dudley and his council "gave no credit to these suspicions" but decided to strengthen the colony's defenses just in case the rumors turned out to be true.
1981, John H. G. Pell, "General George Washington's Visit to Fort Ticonderoga in July 1783", Bulletin of the Fort Ticonderoga Museum, Volume XIV, Number 1, Summer 1981, Fort Ticonderoga Museum, page 260,
  Throughout 1782 and most of 1783 there was a mixture of defacto [sic] peace but preparedness for war just in case it should be resumed, a sort of cold war.