Is “Slackest” a Scrabble word?
Yes
SLACKEST
Valid in at least one Scrabble dictionary
Valid in these dictionaries
SOWPODS / CSW
Scrabble UK & International
TWL / NWL
Scrabble North America
Large English Dictionary
General English word list
Meaning of Slackest
adjective
1.(normally said of a rope) Lax; not tense; not firmly extended.
“a slack rope”
2.Weak; not holding fast.
“a slack hand”
3.Lacking diligence or care; not earnest or eager.
“slack in duty or service”
Synonyms: easy, moderate, slow