Penitence (poenitentia) is named as if it were punishment (punitentia), because by repenting a man punishes the wrong he does. Indeed, those who truly repent do nothing other than make it so that their evil does not go unpunished. In so far as they do not spare themselves, they are spared by He whose high and just judgment no contemptuous person evades. And the complete penitence is to weep (deflere) for those in the past and not allow them to be in the future. It accordingly is like a fountain (in similitude fontis), such that if some sin by the attack of the devil sneaks in (inrepserit), by satisfaction it is cleansed (purgetur). And satisfaction is to exclude the things that cause and suggest sin and beyond that not to repeat the sin. But reconciliation is what is brought about after the fulfillment of penitence. For as we are endeared (conciliamur) to God when we are first converted from heathenry (a gentilitate), so we are reconciled when after sinning we, by repenting, return.
[St. Isidore of Seville, Etymologies VI.19.71-74, my translation. 'Poenitentia' can be translated 'penance', 'penitence', or 'repentance', as you please. 'Deflere' is a very strong word, and could be rendered as 'bewail' rather than 'weep'.]
While St. Isidore doesn't here explicitly say it, he seems clearly to be establishing a parallel between baptism and the sacrament of repentance -- as baptism has the baptismal font, so penance has its font, the tears of repentance; as we are commended or endeared to God by baptism, we are re-commended or re-endeared to God by penance.