all-

From Middle English all- (also al-), from Old English eall-, eal- (“all-”), from Proto-Germanic *ala- (“all-”), from Proto-Indo-European *al- (“all”). Cognate with Dutch al-, German all-, Swedish all-. More at all.

all-

1. Indicates complete power or authority in an area.

   all-knowing

   all-loving

   all-seeing

   all-powerful

2. Indicates that a term applies in a general manner.

   all-around

   all-over

   all-right

• LAL, Lal, Lal.

From all.

all-

1. located beneath, at the bottom, nether, sub-

all-

1. fairly, rather, decently ^syn. [since the 17th century]
2. (dated) very ^syn.

   Ekki allfáir viðskiptavinir.

   Very many customers.

• Hyphenating compound terms prefixed with all- is allowed (same with hálf-, jafn- and lang-):[1]

  All-óhugnanlegur.

  Rather gruesome.

• (rather): ^def. sæmilega, heldur, nokkuð
• (very): ^def. mjög, einkar

• allfár (dated, synonym mjög fár; very few)
• allgóður (dated, synonym mjög góður; very good)
• allgóður (contemporary meaning, synonym sæmilegur; decent)
• allmikill (contemporary meaning; considerable)
• allvel (dated, synonym mjög vel; very well)
• allvel (contemporary meaning, synonym þolanlega, sæmilega; reasonably)
• enginn er allheimskur ef þegja má (no one is completely stupid if they can hold their silence; see silence is golden)

• dá- (“rather, fairly, quite”)

From allr (“all”).

all-

1. used as an intensive in front of adjectives and adverbs; very, extremely

• all- in A Concise Dictionary of Old Icelandic, G. T. Zoëga, Clarendon Press, 1910, at Internet Archive.