"the earth regarded as placed midway between heaven and hell or the abode of the gods and the underworld," late 13c., from middle (adj.) + earth. Altered from earlier middel-erd (late 12c.), midden-erd, itself an alteration (by association with Middle English eard "dwelling") of Old English middangeard (see Midgard).
"room in which food is cooked, part of a building fitted out for cooking," c. 1200, from Old English cycene "kitchen," from Proto-Germanic *kokina (source also of Middle Dutch cökene, Old High German chuhhina, German Küche, Danish kjøkken), probably borrowed from Vulgar Latin *cocina (source also of French cuisine, Spanish cocina), a variant of Latin coquina "kitchen," from fem. of coquinus "of cooks," from coquus "cook," from coquere "to cook" (from PIE root *pekw- "to cook, ripen").
The Old English word might be directly from Vulgar Latin. Kitchen cabinet "informal but powerful set of advisers" is American English slang, 1832, originally in reference to President Andrew Jackson, whose intimate friends were supposed to have more influence with him than his official advisers. Kitchen midden (1863) in archaeology translates Danish kjøkken mødding. Surname Kitchener ("one employed in or supervising a (monastic) kitchen") is from early 14c.