Forewarned is forearmed
Advance warning provides an advantage.
Many idioms that have no obvious source are often referred to, for no good reason, as 'old proverbs'. 'Forewarned is forearmed' has a genuine claim to be called such, as it dates from at least the end of the 16th century, and could be much earlier. The Latin saying 'praemonitus, praemunitus' loosely translates as 'forewarned is forearmed'. There's no evidence to show that the English proverb is merely a translation of the Latin though. The two sayings could easily have originated independently.
The meaning of the proverb is quite straightforward and literal - so long as it is understood that forearm is here the archaic verb meaning 'to arm in advance', rather than the noun forearm, i.e. the part of the arm between the elbow and wrist. The saying is so straightforward in fact that it was originally simply 'forewarned, forearmed'. It is found in that form in Robert Greene's A Notable Discovery of Coosnage (a.k.a. The Art of Conny-catching), 1592:
"forewarned, forearmed: burnt children dread the fire."
The History of New Hampshire was published by John Farmer in 1831. This was a republishing of the text of various letters which were written in the 1680s. It includes 'A letter from Capt. Francis Hooke, advising of danger from the Indians', 1685, which contains another example of the early form of the proverb:
"A word to the wise is enough. The old proverb is, forewarned, forearmed."
The description of the proverb as 'old' at that date does raise the possibility of the 1592 citation not being the earliest.
'Forewarned, forearmed' transformed into the 'forewarned is forearmed' version we know now during the 18th century. Abraham Tucker used that form of the proverb in The Light of Nature Pursued, 1768:
"Knowing that forewarned is forearmed."
Tucker was something of a specialist on the subject - just five years before, he had published a treatise titled Freewill, Foreknowledge, and Fate.
See also - Latin Phrases in English.