What does mean "a" of the prefix?

Question

What does mean "a" of the prefix?

some adjectives or adverbs are "a" + noun

for example
-ablaze
-aground
-afloat
-afoot
etc.

Answer 1

There are several different prefixes that end up as "a-". But the words you mention all happen to use the same one. It does come from an old preposition "on", but that preposition has a range of meanings, and a bit different sense depending on whether it is added to a noun or a verb. It is described in one dictionary as follows:

a-1
a reduced form of the Old English preposition on, meaning “on,” “in,” “into,” “to,” “toward,” preserved before a noun in a prepositional phrase, forming a predicate adjective or an adverbial element (afoot; abed; ashore; aside; away), or before an adjective (afar; aloud; alow), as a moribund prefix with a verb (acknowledge), and in archaic and dialectal use before a present participle in -ing (set the bells aringing); and added to a verb stem with the force of a present participle (ablaze; agape; aglow; astride; and originally, awry).

Note that of your list, two of them are based on VERBS, and fit the last description above, that is the 'a-' is nearly the same as adding -ing to the end. Thus "ablaze" is like "blazing"; "afloat" is like "floating".

The other two you list add the prefix to a NOUN and have the prepositional sense of "on" --'aground' = 'on(to) ground'; 'afoot' = 'on foot' (though this second one has also taken on the meaning 'in progress').

For this and five other "a-" prefixes, their sources (some from Old English others from Latin or Greek) and meanings, see:
http://dictionary.reference.com/browse/a-

Answer 2

Geeze, welcome to the English language and all its inconsistencies.

They mean that something is currently happening.
'Ablaze' means 'something is burning'.
'Aground' means 'that a ship has run into the ground, beached itself or currently on the ground'.
'Afloat' means 'that a ship is currently floating'.

'Afoot' of course is completely out to lunch, since it means 'that something sinister is happening'. Nothing to do with feet in the first place. Makes you wonder why a multitude of sinister things happening wouldn't be called 'Afeet'.

"Doctor Watson, I believe that many things are afeet!"

As I said, welcome to the world of inconsistencies.

Answer 3

1 : on : in : at
2 : in (such) a state or condition
3 : in (such) a manner
4 : in the act or process of

Answer 4

it means the verb is presently happening. e.g:
ablaze=blazing
aground= on the ground
afloat=floating
afoot=happening right now (the game is afoot means the game has started)

Answer 5

in most cases it means "not" or the opposite of the other word being used, here i think it means "on" but im not entirely sure

Answer 6

in the examples you gave, it means "on"