[undies]

U would have to be the easiest letter .......

1 un-
prefix \ˌən, often ˈən before ˈ-stressed syllable\
Definition of un-

1
: not : in-, non- —in adjectives formed from adjectives <unambitious> <unskilled> or participles <undressed>, in nouns formed from nouns <unavailability>, and rarely in verbs formed from verbs <unbe> —sometimes in words that have a meaning that merely negates that of the base word and are thereby distinguished from words that prefix in- or a variant of it (as im-) to the same base word and have a meaning positively opposite to that of the base word <unartistic> <unmoral>

2
: opposite of : contrary to —in adjectives formed from adjectives <unconstitutional> <ungraceful> <unmannered> or participles <unbelieving> and in nouns formed from nouns <unrest>

unzip