firmness that you can lean on: steadiness, reliability, faithfulness
At its root this word means firmness, the quality of something that holds steady and does not give way. In its one most literal use it describes Moses' hands held steady until sundown. Everywhere else that physical picture has become a picture of character: a person or a God who is dependable, who can be counted on to keep doing what is right and to keep their word.
The senses below are not different meanings so much as the same idea pointed in different directions. Steadiness in a pair of arms, steadiness in how a person handles money or speaks the truth, and steadiness in how God treats his people are all the same trait of being firm and trustworthy. The richest cluster of uses is in the Psalms, where God's faithfulness is celebrated as the thing that never fails, paired again and again with his loyal love.
Because the word is about reliability rather than mere sincerity, English translators reach for many words: faithfulness, steadfastness, honesty, truth, stability, even security. Each catches part of it. The common thread is something solid enough to put your full weight on.