H4970 מָתַי (maw-thah'ee)

KJV renderings: long, when.
Strong's definition: properly, extent (of time); but used only adverbially (especially with other particle prefixes), when (either relative or interrogative)
Derivation: from an unused root meaning to extend;

From James Strong's public-domain dictionaries (H), JSON edition by OpenScriptures (CC-BY-SA). Every dotted-underline word in the text links here.