Today's word is NISI, which is a contraction of ne and si, "if not." Unfortunately, people often try to use the archaic English equivalent, "lest," in order to translate Latin nisi. As a general rule, it is far easier to simply translate nisi as "if not" or with the more contemporary English word "unless." You can also find the same idea expressed as si...non with no real difference in meaning.
In addition, you can find nisi used to mean something like "except," or "only," as in this famous saying: Nemo nisi mors, "No one except death" (i.e. "No one except death will part us.")
Here are some examples of today's word in Latin sayings and proverbs; for more information, see the page at the Scala Sapientiae, which contains notes on some of the proverbs cited below, as well as additional proverbs:Nisi causas scimus, nihil scimus.
Nihil nisi mors certum est.
Bonus vir nemo est, nisi qui bonus est omnibus.
Nemo nisi mors.
Nemo nisi suo die moritur.
Nemo autem regere potest, nisi qui et regi.
De vivis nil nisi verum.
Nil magnum nisi bonum.
Nil scio nisi nescio.
Nemo malus nisi probetur.
De absentibus nihil nisi bonum.
De absentibus nisi bene.
Perdes maiora, minora nisi servaveris.
De mortuis nil nisi bonum.
Cum tuus es, noli servire, nisi tibi soli.
Nil, nisi quod prodest, carum est.
Nemo nisi sapiens liber est.
Nemo sapiens nisi patiens.
Nemo ab alio contemnitur, nisi a se ante contemptus.
Nihil pulchrum, nisi utile.