The Spanish word 'septuagésimo' (meaning 'seventieth') comes directly from the Latin word 'septuagesimus', which also meant 'seventieth'. This is one of several Spanish ordinal numbers that preserves its Latin form quite closely, with only minor changes to adapt it to Spanish phonology and spelling conventions - notably the addition of the accent mark and the change from '-us' to '-o' ending, which is typical when Latin words were adopted into Spanish.

Some related and simpler Spanish words include 'setenta' (seventy) and 'siete' (seven). These words share the same Latin root related to the concept of 'seven', though 'septuagésimo' maintains a more formal, learned form closer to the original Latin.

In English, we can find related words like 'septuagenarian' (a person in their seventies) and 'Septuagint' (the ancient Greek translation of the Hebrew Bible, supposedly translated by 70 scholars). These words share the same Latin root 'septuaginta' meaning 'seventy'. While English doesn't commonly use 'septuagesimal' for 'seventieth', these formal terms preserve the same classical root.