The Spanish verb testimoniar ('to testify') comes from the Spanish noun testimonio ('testimony'), which itself derives from the Latin word testimonium with the same meaning. The Latin testimonium was formed by combining testis ('witness') with the suffix -monium (used to form abstract nouns). The Spanish verb was created by adding the common verb-forming suffix -ar to testimonio.

A simpler and more common related Spanish word is testimonio ('testimony'). This noun is used more frequently than the verb testimoniar and might be easier to remember. Another related word is testigo ('witness'), which comes from the same Latin root testis.

English speakers can connect testimoniar to several familiar English words that share the same Latin origin: 'testimony,' 'testify,' and 'witness.' The connection is clear in both form and meaning - just as testimoniar means 'to testify' in Spanish, these English words all relate to giving evidence or bearing witness. The word 'test' is also related, as it originally referred to a vessel used to evaluate the quality of metals - essentially serving as a 'witness' to their purity.