Check out this Wikipedia entry on § 67(a), the 2% floor on miscellaneous itemized deductions: The Two-Percent Haircut:

The Two-Percent Haircut^[1], otherwise known as the two-percent floor, is a limitation on miscellaneous itemized income tax deductions and is codified under Internal Revenue Code.  I.R.C. § 67(a). …

[Fn.1: Samuel A. Donaldson (Washington), Federal Income Taxation of Individuals: Cases, Problems, and Materials 30 (Thomson-West, 2d ed. 2007).]

The policy reason for the two-percent haircut is quite evasive. We may be able to gauge Congress' intent with the two-percent haircut by looking at the possible reasons for including certain items. The most significant expense that is categorized as a miscellaneous itemized deduction is the unreimbursed business expenses of an employee. It is a possibility that Congress imposed the two-percent haircut on these expenses in order to weed out portions that may have been personal in nature, as major employee expenses are generally reimbursed by the employer anyway. However, this policy rationale doesn't make much sense when one considers Congress' unlimited allowance of many personal expense deductions (some above-the line). The two-percent haircut is a thing of mystery.

(Hat Tip: Lauren Jones.)