sagaca

Two letters apart from saĝa, and not related to it at all. Saĝa came in through Italian saggio and French sage, from a Vulgar Latin sapius built on sapere, to taste and thence to know. Sagaca came in from Latin sagax, keen-scented, from sagire, to catch a scent. Two different Latin roots, two different metaphors for intelligence - one from the tongue and one from the nose - and they happen to collide in Esperanto at sag-. The noun pair is the giveaway when you need to check yourself: saĝeco is wisdom, sagaco is shrewdness, and they are not interchangeable.

Register: this is a literary word and it should feel like one. Saĝa is everyday and you will hear it in a kitchen. Sagaca belongs in prose, and it carries a specific flavour that saĝa does not: quickness at reading a situation, especially one where somebody may be trying it on. A saĝa person gives you good advice; a sagaca person notices what you left out. Zamenhof's own translations use it sparingly, and so should you.

Where it sits among its neighbours. Lerta is skilful with the hands or with a task. Ruza is cunning with a moral debit attached - a fox is ruza, and you would not want it said of you. Klera is well-read, which is knowledge rather than judgement. Sagaca is the one with no debit and no bookshelf: it is judgement, sharp and fast.