Emmett began as a surname connected to the Germanic Emma, itself tied to ideas of wholeness or universality, before settling into use as a given name in its own right. Its meaning leans toward completeness -- someone regarded as sufficient in himself rather than defined by a single quality.
Mercury in the sixth house of a nativity, the native shall change from one religion to another, & will have his felicity in part impedited by reason of his inconstancy. - William Lilly (1602.-1681.)