w3 と lookup が必要ですが、英辞郎 on the Web を引いてみますた。
http://www.alc.co.jp/

(defun eijiro (pattern &optional mode)
"Look up PATTERN in the online Eijiro dictionary.
MODE should be \"ej\" for English-Japaneseor
or \"je\" for Japanese-English."
(interactive (lookup-pattern-input))
(let ((cgi "http://home.alc.co.jp/db/owa/eijiro_red2")
(word-prefix "word_in=")
(mode-prefix "type_in=")
;(url)
;(hexified-pattern)
(top-margin 15))
(or mode (setq mode "ej"))
(setq hexified-pattern (url-hexify-string pattern))
(setq url (concat cgi "?" word-prefix
hexified-pattern "&" mode-prefix mode))
(w3-fetch url)
(message (concat "Looking up : " pattern ))
(set-window-point (selected-window) (point-min))
(scroll-up top-margin)))

英日/日英 自動切換えはまだです…
(やりたいという意思表示は &optional mode あたりに…)
xemacs 21.4 (patch 6) でしか確認してません。