Psa. 78:37 KJV (With Strong’s)

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37
For their heart
leb (Hebrew #3820)
the heart; also used (figuratively) very widely for the feelings, the will and even the intellect; likewise for the centre of anything
KJV usage: + care for, comfortably, consent, X considered, courag(-eous), friend(-ly), ((broken-), (hard-), (merry-), (stiff-), (stout-), double) heart((-ed)), X heed, X I, kindly, midst, mind(-ed), X regard((-ed)), X themselves, X unawares, understanding, X well, willingly, wisdom.
Pronounce: labe
Origin: a form of 3824
was not right
kuwn (Hebrew #3559)
properly, to be erect (i.e. stand perpendicular); hence (causatively) to set up, in a great variety of applications, whether literal (establish, fix, prepare, apply), or figurative (appoint, render sure, proper or prosperous)
KJV usage: certain(-ty), confirm, direct, faithfulness, fashion, fasten, firm, be fitted, be fixed, frame, be meet, ordain, order, perfect, (make) preparation, prepare (self), provide, make provision, (be, make) ready, right, set (aright, fast, forth), be stable, (e-)stablish, stand, tarry, X very deed.
Pronounce: koon
Origin: a primitive root
with him, neither were they stedfast
'aman (Hebrew #539)
properly, to build up or support; to foster as a parent or nurse; figuratively to render (or be) firm or faithful, to trust or believe, to be permanent or quiet; morally to be true or certain; once (Isa. 30:21; interchangeable with 541) to go to the right hand
KJV usage: hence, assurance, believe, bring up, establish, + fail, be faithful (of long continuance, stedfast, sure, surely, trusty, verified), nurse, (-ing father), (put), trust, turn to the right.
Pronounce: aw-man'
Origin: a primitive root
in his covenant
briyth (Hebrew #1285)
a compact (because made by passing between pieces of flesh)
KJV usage: confederacy, (con-)feder(-ate), covenant, league.
Pronounce: ber-eeth'
Origin: from 1262 (in the sense of cutting (like 1254))
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Cross References

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their heart.
stedfast.

J. N. Darby Translation

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37
For their heart was not firm toward him, neither were they stedfast in his covenant.